1. Field of the Invention
The present invention relates to a chemically amplified resist composition, and more particularly, to a photosensitive polymer having a cyclic backbone, and to a resist composition for an ArF excimer laser obtained therefrom.
2. Description of the Related Art
As semiconductor devices become highly integrated, fine pattern formation is required from photolithography processes used in the fabrication of such devices. Further, as the capacities of semiconductor devices increase beyond 1 giga bit, a pattern size having a design rule of less than 0.2 μm becomes necessary. This places limitations on the use of conventional resist materials adapted for the KrF excimer laser (wavelength: 248 nm). Thus, to permit a lower wavelength operation, new resist materials capable of being developed using an ArF excimer laser (wavelength: 193 nm) have been developed for use in lithography processes.
Present resist materials that are commercially used in lithography processes employing the ArF excimer laser suffer certain drawbacks when compared with more conventional resist materials. The most common problems relate to transmittance of the polymer and resistance to dry etching.
As widely known ArF resist materials, (meth)acrylate polymers are generally used. In particular, the most common resist material is poly(methyl methacrylate-tert-butyl methacrylate-methacrylic acid) terpolymer system manufactured by IBM, Inc. However, such polymers have very weak resistance to dry etching.
Accordingly, to increase the resistance to dry etching, a polymer having a backbone composed of an alicyclic compound such as an isobornyl group, an adamantyl group or a tricyclodecanyl group, is used. However, the resulting resist still exhibits weak resistance to dry etching.
Also, since the alicyclic compound is hydrophobic, in the case where the alicyclic compound is contained in the terpolymer, the adhesion to underlying layers is deteriorated.
In an attempt to overcome the above-described problem, a tetrapolymer represented by the following formula in which a carboxylic acid group is introduced to the backbone of the polymer has been proposed (see J. Photopolym. Sci. Technol., 7(3), 507 (1994).). 
However, the resist layer obtained from the polymer having the above structure still has poor adhesion to underlying layers, and resistance to dry etching is poor. Also, a developing solution that is generally usable for development must be diluted before being used.
Alternatively, a methacrylate copolymer having an alicyclic protecting group represented by the following formula has been proposed (see J. Photopolym. Sci. Technol., 9(3), 509 (1996).). 
The resist layer obtained from the polymer having the above structure still has poor resistance to dry etching. Also, severe line edge roughness is observed when a line pattern is formed from the resist layer. Also, the manufacturing cost for raw materials for preparing the copolymer is very high. In particular, in order to improve an adhesion characteristic, a monomer having a lactone group is introduced thereto. However, the monomer generally to costly for practical use. Thus, it is desirable to introduce a new monomer with which an expensive monomer can be replaced to facilitate commercial use as a resist material.
As another conventional polymer, a cycloolefine-maleic anhydride (COMA) alternating polymer represented by the following formula has been proposed (see J. Photopolym. Sci. Technol., 12(3), 501 (1999).). 
The resist layer obtained from the polymer having the above structure is poor in terms of resolution, transmission, adhesion characteristic and yield. Also, due to the structural characteristic of the backbone, the resist layer has a high glass transition temperature of about 200° C. or higher. Thus, several problems may be encountered in processes using the resist layer.